Research on Small Matter Makes Big Headlines
Two Rensselaer researchers have made groundbreaking developments in growing and discovering new properties of carbon nanotubes. Their unprecedented research has been highlighted in some of the most noted scientific periodicals in the nation, including Nature and Science magazines.

Pulickel Ajayan, professor, and Ganapathiraman Ramanath, assistant professor, both of materials science, have learned how to grow carbon nanotubes in near-limitless ways. They also have discovered that the nanoscopic cylinders will ignite under certain circumstances.

Growing Nanotubes Every Which Way

Next-generation computer chips, integrated circuits, and the microelectro-mechanical (MEMS) devices that power them depend upon carbon nanotubes that can be grown up, down, sideways, and in all three dimensions. Ajayan (above) and Ramanath (right) are the first to achieve this unprecedented, specific, and controlled nanotube growth.

Their research, reported in the April 4 issue of the journal Nature, paves the way for Lilliputian devices that depend on tiny networks and architectures.

The method is based on a selective growth process that allows the nanotubes to grow perpendicular to the silica-coated substrate. By chiseling the silica into predetermined shapes, Ajayan and Ramanath are able to precisely control and direct the nanotube growth. Their use of gas phase delivery of a metal catalyst, essential for nanotube growth, makes their growth process more flexible and more easily scalable than conventional methods.

This simple process for controlled nanotube growth could be brought to market in a matter of months, the researchers say.

The impact of our work is well beyond nanotubes, Ramanath says. This is the first step toward making complex networks comprised of molecular units. By manipulating the topography of the silica blocks, and utilizing the selective and directional growth process, we have been able to force nanotubes to grow in predetermined, multiple directions, with a very fine degree of control. No one else has done this.

The researchers work is funded by the Office of Naval Research and the Interconnect Focus Center.

Carbon Nanotubes Ignite

Ramanath and Ajayan also have discovered a surprising new property of single-walled carbon nanotubes. When exposed to a conventional photographic flash, the nanotubes emit a loud pop and then ignite. The discovery, reported in the April 26 issue of the journal Science, could mean that the nanotubes might be used in light sensors or to remotely trigger explosives and combustion reactions, although researchers say that more testing needs to be done to realize these possibilities.

Carbon nanotubes emit a loud pop and then ignite when exposed to a conventional photographic flash.

The researchers explain that the loud popping sound heard after the flash is a well-known phenomenon, called the photo acoustic effect. It occurs when porous black objects, such as carbon nanotubes, absorb a large amount of light, which results in the expansion and contraction of the gas surrounding them, releasing sound.

What surprised the researchers, however, was that the nanotubes then spontaneously ignited.

The single-walled carbon nanotube samples in this situation were just a jumble of tubes. They were not laid out in any pattern, and because of that, the heat generated from the flash could not dissipate, so the nanotubes just burned, Ajayan says.

The discovery was initially noted by Andres de la Guardia when he took flash photographs of the nanotubes. De la Guardia is a graduate student in operations research and statistics.

Since the discovery, the researchers have found that while the tubes burned only when oxygen is present, their atomic structure was altered even in inert gas environments.

From an applications perspective, our work opens up exciting possibilities of using low-power light sources to create new forms of nanomaterials, and will serve as a starting point for developing nanotube-based actuators and sensors that rely on remote activation and triggering, says Ramanath.

The research is a collaborative effort between Rensselaer, a French group headed by T.W. Ebbesen, and researchers in France, Mexico, and Germany.